1. Field of the Invention
This invention relates generally to television receiver systems and, more particularly, to an improved horizontal phase lock loop which employs an oscillator which oscillates at twice line frequency, provides an accurate fifty percent duty cycle horizontal output drive waveform and provides an accurate fifty percent duty cycle phase detector switching waveform to assure symmetrical pull-in performance.
2. Description of the Prior Art
In most modern television receivers, a phase lock loop is employed to lock the horizontal oscillator in the horizontal sweep section of the receiver to the incoming separated horizontal synchronization pulses to establish the phase relationship between the incoming synchronization pulses and the horizontal flyback pulse thus fixing the television picture on the raster. In the prior art single loop system, the horizontal oscillator operates at line frequency (15.75 KHz in the United States) and its output is a ramp signal. This ramp signal is then shaped to form a square wave drive signal to the horizontal output stage which in turn produces flyback pulses approximately 10 microseconds wide and approximately 63 microseconds apart. The flyback pulses are then processed by external wave shaping circuitry to produce a ramp signal which is applied to one input of a horizontal phase detector. The phase detector has a second input coupled to a bias voltage. The incoming horizontal synchronization pulse gates the phase detector which generates a d.c. output for controlling the horizontal frequency.
This system suffers from several disadvantages. First, many external components are required to shape the flyback pulse stream into a ramp for the phase detector. Second, the pull-in symmetry of the loop depends on the shaped ramp and the flyback signal which may vary with signal conditions and loading. Third, the loop's response time or bandwidth is a compromise between a requirement for good low signal operation (low bandwidth and slow response time) and variations in loop delay in the horizontal output stage which requires a fast loop response to avoid horizontal displacement of portions of the picture on the raster. Finally, this system does not provide a clock signal at twice line frequency for a vertical countdown section. If the vertical deflection signal is derived from the horizontal oscillator by countdown, then the twice line frequency clock is required at the counter input to provide for proper vertical interlacing.
To reduce the effects of the first three disadvantages described above, two loop systems have been employed; e.g. the TBA920/TDA2590 2-loop system by Philips. This system suffers, however, from the disadvantage that the duty cycle of the horizontal output stage drive signal varies as the second loop corrects for changes in loop delays. Furthermore, this system does not employ a 31.5 KHz oscillator necessary for vertical countdown; i.e. twice U.S. line frequency.